It is often advantageous to insulate certain types of surfaces against potentially damaging environmental influences and improper handling by overcoating them with a protective layer. In this regard, the application of a protective overcoat onto such a surface by means of lamination has become a favored practice. One surface that benefits from such lamination is an image surface of a thermal imaging medium, such as described in International Patent Application No. PCT/US87/03249 (Publication No. WO 88/04237) (Etzel), the image surface being formed after imagewise formation of the medium's image-forming layer.
More particularly, International Patent Application No. PCT/US87/03249 describes a thermal imaging medium and a process for forming an image in which a layer of a porous or particulate imaging material (preferably, a layer of carbon black) is deposited on a heat-activatable image-forming surface of a first sheet-like element, the layer having an adhesive strength to the first sheet-like element that is a function of its exposed state. Portions of this thermal imaging medium are then exposed to brief and intense radiation (for example, laser scanning), to firmly attach exposed portions of the imaging material to the first sheet-like element. Finally, those portions of the imaging material not exposed to the radiation (and thus not firmly attached to the first sheet-like element) are removed, thereby forming a binary image surface comprising a plurality of first areas where the imaging material is adhered to the first sheet-like element and a plurality of second areas where the first sheet-like element is free from the imaging material.
In an embodiment of the thermal imaging medium described by International Patent Application No. PCT/US87/03249, a first sheet-like element of imaging material is covered with a second sheet-like element that is laminated to the first so that the imaging material is confined between the first element and the second element. After imaging and separation of the second element (with the unexposed portions of the imaging material) from the first element, a pair of image surfaces is obtained. The first image surface comprises exposed portions of image-forming substance more firmly attached to the first element by heat activation of the heat-activatable image-forming surface. The second image surface comprises non-exposed portions of the image-forming substance carried or transferred to the second sheet element. Either of the pair of image surfaces may, for reasons of informational content, aesthetic or otherwise, be considered the principal image surface, and all of the following discussion is applicable to both types of image surface.
While the image-forming process described in International Patent Application No. PCT/US87/03249 is capable of producing high quality, high resolution images, the durability of the image surfaces produced by this process may be inappropriate for certain desired applications. In the finished image surface, the porous or particulate imaging material, typically carbon black admixed with a binder, lies exposed (unprotected). The image may be vulnerable to being smeared, damaged or removed by, for example, fingers or skin surfaces (especially if moist), solvents or friction during manual or other handling of the image.
Given such opportunities for mishandling and the like, it may be desirable to protect the image surface of the developed thermal imaging medium by the application of a protective overcoat, e.g. a thin, transparent, but durable layer and adhesive, such as described in International Patent Application No. PCT/US91/08345 (Publication No. WO 92/09930) (Fehervari et al.); and pending U.S. application Ser. No. 08/065345 (Bloom et al.).
Lamination of protective overcoats, such as those described in the cited patent applications, has been accomplished using a continuous roll, i.e. web, to conduct the durable layer to the image carrying sheets, the durable layer typically being associated with an adhesive layer. Activation energy to fuse the durable layer to the imaged sheet is provided at a nip existing between paired compression rollers. Generally, the top roller is actively heated and the bottom roller (by conduction from the top roller) reaches a steady state temperature well above room temperature. Lamination is effected by the cooperative influences of both compression and thermal heating. While such method has provided good results, rippling and curling of the laminated image sheets is problematic especially for large scale laminations, e.g. 14 by 17 inches more or less.
In light of the above, need exists for a laminating system and method useful for laminating a protective overcoat onto a receiving surface while eliminating or at least minimizing ripple and curl of the laminated sheets, and it is a primary object of the invention to provide a solution to the foregoing problems.
Other objects of the invention will, in part, be obvious and, in part, appear hereinafter when the following detailed description is read in conjunction with the drawings.